1. Field of the Invention
The present invention relates generally a read head for reading data from magnetic media and in particular to a magnetoresistive read head and a method for obtaining position error information from this read head.
2. Description of the Related Art
Magnetic media, such as magnetic tapes and disks, are recorded with data, such as computer data, music, video, etc. The recorded data are read from the magnetic media by a read head, which passes over the magnetic media to detect changes in the magnetic fields recorded on the magnetic media. Read heads, sometimes referred to as playback heads, may be single read heads or multitrack read heads. In disk recording, most read heads are single read heads, and the disk drive will typically use several single read heads to increase reading speed. In tape drives, most read heads are multitrack read heads, and the drive typically will use one multitrack read head.
As the data density on tapes and disks has increased, the tape- and disk drive typically will employ some type of fine positioning of the head relative to the tape or disk based on the feedback of position information from the media (position error signal—PES). This is typically denoted tracking servo. Several methods exist to generate PES from tape or disk.
U.S. Pat. No. 5,689,384 describes an arrangement for timing-based PES generation wherein the time between the output pulses of the head is measured and the position of the head relative to the tape is determined from this timing. U.S. Pat. No. 5,426,543 describes an arrangement for amplitude-based PES generation wherein the output of the head is amplitude modulated, and wherein the amplitude modulation is a measure of the position of the tape relative to the head. Both of these systems for generating PES have the disadvantages that they a) require magnetic pre-formatting at the media manufacturer, which is a cost concern b) have inaccuracies associated therewith that may cause a tracking error when the track width becomes small.
The data may be recorded on the magnetic media in adjacent tracks simultaneously. A write head for writing such tracks is described in U.S. Pat. No. 6,650,496. Because the data are written on all the adjacent tracks in the track set at the same time, the read head required to read such track set includes multiple read sensors positioned very close to each other or a contiguous sensor element and precise positioning relative to the tracks. U.S. Pat. Nos. 5,920,538 and 6,771,567 are examples of read heads for reading such adjacent tracks. A method and apparatus for preventing crosstalk between sensors positioned to sense data from such consecutively recorded parallel tracks is described in U.S. Pat. No. 5,493,553. Such a cross-talk cancellation technique has serious limitations at very narrow track widths: During the writing of the data, the edge of the track always will be written imperfectly due to the field gradient around the corner of the pole pieces of the head. The write field distribution and the field strength are different at the edge of the head than in the center of the head. Because of this, there is always a “side write” or “erase” band at the track edge, which to a certain degree is a source of noise during playback if the read head is sensing it. In order to have cross-talk between two adjacent readers as described in U.S. Pat. No. 5,493,553 when the tracks are written as described in U.S. Pat. No. 6,650,496, the read heads therefore also must sense the noise from the “side write” or “erase” band, which is a serious disadvantage when reading narrow tracks. Such erase bands are described, for instance, in the article “Tape Erase Bands Measured by MFM Compared to MR Head Cross Erase Band Responses,” T. Pan et. al., IEEE Trans. Magnet., Vol. 32, No. 5, pp. 3407-3409 (1996).
Read heads based on the magneto-resistive (MR) effect are well known technology for reading recorded data and servo information on a magnetic media. A large number of patents, articles and books have been published on the subject, and a number of products are based on this technology. A possible solution for reading data from adjacent tracks is to use a MR head having a single bias current to bias two or more MR sensors at the same time (“comb head”) as described in the article “Multi-Tapped Magnetoresistive Heads for Magnetic Tape Tracking Servo,” presented at Joint Intermag/MMM 1998 Conf., San Francisco, Calif. 1998 (Paper AE-07) and published in revised form in IEEE Trans. Magnet., Vol. 34, No. 4, pp. 1904-1906 (7/1998). Using this configuration it is possible to sense, as described in the article, two track edges at the same time differentially to obtain position information from a simple servo pattern written on the tape. It is clear that the same head also can be used to sense data information from two or more adjacent tracks, but the read sensors also would sense the noise from the erase bands between the tracks, which is problematic for the data decoding, in particular modern detection methods like Partial Response Maximum Likelihood (PRML).